The invention concerns a charging material preheater for preheating charging material which is to be charged into a furnace vessel.
A charging material preheater of that kind is described for example in WO-A1-95/04910. An advantageous use of a charging material preheater of the general kind set forth is described in WO-A-90/10086. Here, an outer segment of the vessel cover of an arc furnace is replaced by a shaft which is fixed in a holding structure and through which the hot furnace gases can be passed. In heat-exchange relationship, they heat the charging material which is disposed in the shaft, and make it possible to achieve a substantial energy saving. The cross-section of the charging material preheater in shaft form can be round or oval with a single shaft wall. Preferably it is of quadrangular, that is to say polygonal cross-section, so that the receiving space for the charging material to be heated is defined by four shaft walls.
The shaft wall or the shaft walls of known charging material preheaters are either formed from refractory material such as refractory bricks, a refractory spray material or a refractory casting material, or however from water-cooled steel wall elements, preferably in the form of tubular panels.
If the shaft walls, on the inward side that is towards the internal space of the charging material preheater, comprise refractory material, then, when a mechanical loading is involved as occurs when the shaft is loaded, that inward side is subjected to a greater degree of wear and the risk of a higher level of damage than water-cooled steel wall elements. For that reason, and also for reasons of weight, operators have changed over to making the shaft walls in the form of fluid-cooled steel walls, in particular in the form of tubular panels which can be connected to a cooling circuit.
As already mentioned, the insides of the shaft walls, in the loading operation involving charging material, are exposed to high levels of mechanical loading, in particular when heavy scrap is also used as the charging material. If the heavy scrap contains for example railway rails which have been cut and broken up into pieces, the sharp edges of such rail pieces, upon being emptied into the shaft from a charging basket which is moved into a position above the upper loading opening of the shaft, break relatively large pieces out of the inside of the wall, in the case of a shaft wall comprising refractory material. Even in the case of tubular panels which have a substantially higher level of mechanical load-bearing capability, serious damage such as leaks can occur due to such loadings.
Although basically the risk of damage to the inside of the shaft walls is greater in the lower region of the shaft than in the upper region, because in the lower region the kinetic energy of the pieces of scrap which are falling from above into the shaft is greater, it is not specifically possible to predetermine at what locations damage which has to be repaired will occur in operation, so that it is also not possible to obviate the need for local repair to the inside of the shaft wall by virtue of precautionary strengthening at such locations.
If a repair becomes necessary, that involves relatively long stoppage times for the charging material preheater, particularly in the case of refractory walls, because of the necessary cooling-down time. In addition, eliminating leaks in the case of water-cooled galls requires unacceptably long stoppage times because of the need to shut down the water circuit and because of the necessary welding operations.
If fluid-cooled steel walls which can withstand a mechanically higher loading are used, that involves energy losses, in comparison with walls in which the inside has a refractory cladding. For shaft cooling of a medium-size furnace, about 700 m3 of cooling water per hour is required. On average the cooling water is heated by 5xc2x0 C. to 6xc2x0 C. It can be deduced therefrom that, with 75 t of liquid steel which are produced In 45 minutes, the average energy loss in the cooling water is about 3360 kWh, that is to say about 45 kWh per ton of liquid steel. In regard to a reduction in the level of energy losses, it would be desirable to replace the more robust fluid-cooled steel walls by shaft walls of refractory material. The obstacle to that is a greater susceptibility to repair and longer stoppage times.
U.S. Pat. No. 3,632,094 A discloses a charging material preheater comprising two shaft-form units which are of a similar structure and which can be fixed on both sides and symmetrically to a central portion which has a gas-permeable separating wall and which is rotatable through 180xc2x0. To reduce the amount of wear and abrasion, that arrangement is intended to ensure that the charging material does not have to be moved from a charging opening present in the upper region through the entire shaft to a discharge opening in the lower region of the shaft, as in the arrangement provides that different kinds of charging material can be heated in two stages in the two portions, before the charging material is transported to the smelting vessel and charged therein.
For operation of the known charging material preheater, it is necessary for the arrangement to have a central portion which is rotatable through 180xc2x0, with a gas-permeable separating wall, and a device which after emptying of the unit performs the rotary movement. Having regard to the high level of mechanical loading involved in the rotational procedure, the known charging material preheater is only suitable for preheating relatively small amounts of charging material.
The object of the invention is to reduce the repair and stoppage times and therewith also the costs in regard to local damage requiring repair at the inside of the shaft, in particular in relation to shaft walls of refractory material. The invention further seeks to provide that the energy losses are reduced in comparison with a shaft comprising fluid-cooled steel walls.
That object is attained by the present invention. Advantageous configurations of the invention are to be found in the preferred embodiments of the invention.
In the charging material preheater according to the invention the shaft wall or, when the shaft is of a polygonal cross-section, the individual shaft walls, is or are subdivided into shaft wail portions which are individually replaceably fixed in a frame structure. That makes it possible on the one hand for the shaft, at the inside at individual locations at which a lower level of mechanical loading is to be expected, in particular in the upper region, to be lined with refractory material or to be formed from refractory plates, while more robust steel wall elements are used at locations which are subjected to more severe loadings. On the other hand, it permits damaged shaft wall portions to be rapidly changed, irrespective of whether this involves a shaft wall portion of refractory material, that is in need of repair, or a leaking tubular panel. Preferably, in regard to the aspect of storage of replacement shaft wall portions, they are of such dimensions that the number of different sizes is minimized.
Shaft wall portions that have proven to be particularly advantageous are those which, at their outside, have support surfaces with which they bear against corresponding co-operating counterpart mountings of the frame structure, when they are fitted into the frame structure. The frame structure is preferably mounted in such a way that it can be raised and lowered in a holding arrangement, by means of which the shaft can be pivoted or moved to the side from a position above a furnace vessel. If a damaged shaft wall portion has to be replaced, then the shaft is moved to the side out of the operating position above the furnace vessel and the damaged shaft wall portion is replaced by a new or repaired one. After the shaft has been moved back again, the charging material preheater is again ready for operation.
Shaft wall portions comprising fluid-cooled steel wall elements are to be capable of being individually taken out of the cooling circuit, to speed up the replacement operation.